In vertebrate animals, the heart is a hollow muscular organ having four pumping chambers as seen in FIG. 1: the left and right atria and the left and right ventricles, each provided with its own one-way valve. The natural heart valves are identified as the aortic, mitral (or bicuspid), tricuspid and pulmonary, and are each mounted in an annulus comprising dense fibrous rings attached either directly or indirectly to the atrial and ventricular muscle fibers. Each annulus defines a flow orifice.
The atria are the blood-receiving chambers, which pump blood into the ventricles. The ventricles are the blood-discharging chambers. A wall composed of fibrous and muscular parts, called the interatrial septum separates the right and left atriums (see FIGS. 2, 3 and 4). The fibrous interatrial septum is a materially stronger tissue structure compared to the more friable muscle tissue of the heart. An anatomic landmark on the interatrial septum is an oval, thumbprint sized depression called the oval fossa, or fossa ovalis (shown in FIG. 4).
The synchronous pumping actions of the left and right sides of the heart constitute the cardiac cycle. The cycle begins with a period of ventricular relaxation, called ventricular diastole. The cycle ends with a period of ventricular contraction, called ventricular systole. The four valves (see FIGS. 2 and 3) ensure that blood does not flow in the wrong direction during the cardiac cycle; that is, to ensure that the blood does not back flow from the ventricles into the corresponding atria, or back flow from the arteries into the corresponding ventricles. The mitral valve is between the left atrium and the left ventricle, the tricuspid valve between the right atrium and the right ventricle, the pulmonary valve is at the opening of the pulmonary artery, and the aortic valve is at the opening of the aorta.
FIGS. 2 and 3 show the anterior (A) portion of the mitral valve annulus abutting the non-coronary leaflet of the aortic valve. The mitral valve annulus is in the vicinity of the circumflex branch of the left coronary artery, and the posterior (P) side is near the coronary sinus and its tributaries.
The mitral and tricuspid valves are defined by fibrous rings of collagen, each called an annulus, which forms a part of the fibrous skeleton of the heart. The annulus provides peripheral attachments for the two cusps or leaflets of the mitral valve (called the anterior and posterior cusps) and the three cusps or leaflets of the tricuspid valve. The free edges of the leaflets connect to chordae tendineae from more than one papillary muscle, as seen in FIG. 1. In a healthy heart, these muscles and their tendinous chords support the mitral and tricuspid valves, allowing the leaflets to resist the high pressure developed during contractions (pumping) of the left and right ventricles.
When the left ventricle contracts after filling with blood from the left atrium, the walls of the ventricle move inward and release some of the tension from the papillary muscle and chords. The blood pushed up against the under-surface of the mitral leaflets causes them to rise toward the annulus plane of the mitral valve. As they progress toward the annulus, the leading edges of the anterior and posterior leaflet coapt and form a seal, closing the valve. In the healthy heart, leaflet coaptation occurs near the plane of the mitral annulus. The blood continues to be pressurized in the left ventricle until it is ejected into the aorta. Contraction of the papillary muscles is simultaneous with the contraction of the ventricle and serves to keep healthy valve leaflets tightly shut at peak contraction pressures exerted by the ventricle. The remaining cardiac valves operate in a similar fashion.
Various surgical techniques may be used to repair a diseased or damaged valve. In a valve replacement operation, the damaged leaflets are typically excised and the annulus sculpted to receive a prosthetic valve. Due to aortic stenosis and other heart valve diseases, thousands of patients undergo surgery each year wherein the defective native heart valve is replaced by a prosthetic valve (either bioprosthetic or mechanical). Another, less drastic, method for treating defective valves is through repair or reconstruction, which is typically used on minimally calcified valves. One problem with surgical therapy is the significant insult it imposes on chronically ill patients and the associated high morbidity and mortality rates associated with surgical repair.
When a valve is replaced, surgical implantation of the prosthetic valve has typically required an open-chest surgery, during which the heart is stopped and the patient is placed on cardiopulmonary bypass (a so-called “heart-lung machine”). In one common surgical procedure, the diseased native valve leaflets are excised and a prosthetic valve is sutured to the surrounding tissue of the valve annulus. Because of the trauma associated with the procedure and the attendant duration of extracorporeal blood circulation, mortality rates during surgery or shortly thereafter typically have been high. It is well established that risks to patients increase with the duration of extracorporeal circulation. Due to such risks, a substantial number of patients with defective valves are deemed inoperable because their condition is too frail to withstand the procedure. By some estimates, up to about 50% of patients suffering from aortic stenosis and who are older than 80 years cannot undergo surgery for aortic valve replacement using conventional open-chest surgery.
Because of drawbacks associated with conventional open-heart surgery, percutaneous and minimally-invasive surgical approaches are garnering intense attention. Minimally invasive surgical techniques have been and continue to be developed. In successfully performed minimally invasive techniques, a conventional sternotomy can be avoided. Access to the heart can be by way of upper sternotomy or thoracotomy allowing a smaller incision and typically shorter healing times, as well as less pain for the patient. Blood loss is typically lower with minimally invasive techniques, hospital stays are shorter, and there may be lower morbidity and mortality rates as compared to conventional surgical techniques.
To obtain at least some of the potential benefits of the smaller incisions required by minimally invasive surgical techniques, prosthetic valves compatible with such techniques are needed. For instance, U.S. Pat. No. 5,411,522 to Andersen et al. describes a collapsible valve percutaneously introduced in a compressed state through a catheter and expanded in the desired position by balloon inflation.
Although such remote implantation techniques have shown great promise for treating certain patients, replacing a valve via surgical intervention is still the preferred treatment procedure. One hurdle to the acceptance of remote implantation is resistance from doctors who are understandably anxious about converting from an effective, if imperfect, regimen to a novel approach that promises great outcomes but is relatively foreign. In conjunction with the understandable caution exercised by surgeons in switching to new techniques of heart valve replacement, regulatory bodies around the world are moving slowly as well. Numerous successful clinical trials and follow-up studies are in process, but much more experience with these new technologies will be required before they are widely accepted. Additionally, the long-term durability of remotely implanted devices is unknown.
In another approach, a flexible heart valve especially suitable for implanting in the aortic annulus has been proposed in U.S. Pat. No. 6,558,418 to Carpentier, et al., and U.S. Pat. No. 6,376,845 to Marquez, et al. More particularly, Carpentier and Marquez disclose single and multi-element frame-and-stent assemblies that include flexible cusps between adjacent commissure portions extending therefrom. A suture-permeable connecting band attached to the disclosed prosthetic valve follows the shape of (i.e., is coextensive with) the underlying frame. In the Carpentier and Marquez approach, the valve is secured by attaching the connecting band (and thereby, the entire contour of the underlying frame, including the cusp and commissure portions) to the surrounding natural tissue. Although this approach represents an advancement of surgically implantable valves, the commissure portions of the frame remain fixedly attached to, and cannot move independently of, the tissue since the sewing band is coextensive with the undulating frame. In addition, suturing the complex, undulating periphery of the sewing band can be difficult and time consuming, as various parts of the valve can interfere with access to the sewing band. Although the valves disclosed in the '418 and '845 patents could be collapsed and inserted through a small incision, such as a thoracotomy, it would be difficult to suture them to the native annulus through such a small incision due to the configuration of the sewing band.
Accordingly, there remains a need for an improved prosthetic heart valve that facilitates placement through small incisions, facilitates easier suture tying at the implantation site, and provides improved hemodynamics. In addition, devices for, and associated methods of, implanting such improved prosthetic valves in a body lumen are also needed, especially a more efficient procedure that reduces the duration a patient needs extracorporeal circulation to undergo a cardiac valve replacement.